1. Field of the Invention
The present invention relates to method and apparatus for buoyancy distribution of offshore deepwater structures, in particular, but not restricted to, buoyancy distribution along a substantially vertical submarine structure, such as a riser, a bundle of risers, or any other structural member.
2. Description of the Prior Art
The structure may form part of a so-called hybrid riser, having an upper and/or lower portions (“jumpers”) made of flexible conduit. U.S. Pat. No. 6,082,391 (Stolt/Doris) proposes a particular Hybrid Riser Tower consisting of an empty central core, supporting a bundle of riser pipes, some used for oil production some used for water and gas injection. This type of tower has been developed and deployed for example in the Girassol field off Angola. Insulating material in the form of syntactic foam blocks surrounds the core and the pipes and separates the hot and cold fluid conduits. Further background has been published in paper “Hybrid Riser Tower: from Functional Specification to Cost per Unit Length” by J-F Saint-Marcoux and M Rochereau, DOT XIII Rio de Janeiro, Oct. 18, 2001. Updated versions of such risers have been proposed in WO 02/053869 A1. The contents of all these documents are incorporated herein by reference, as background to the present disclosure.
Buoyancy of offshore structures is achieved by using temporarily or permanently attached buoyancy modules providing an upward thrust when submerged in the sea. Conventional devices such as stop-collars and clamps are used to transmit the buoyancy thrust from the buoyancy modules to the supported structure. The buoyancy thrust acts upon the structure where it is generated. The buoyancy modules are clamped around stop collars using straps or bolts.
In particular cases, such as a hybrid riser tower (bundle of risers, fabricated onshore), buoyancy may be required for the supporting of a structure in two (or more) completely different orientations, such as a horizontal orientation (during installation) and a vertical orientation (in operation).
The buoyancy thrust has to be transmitted in both orientations along two perpendicular directions, depending upon the orientation of the structure at the time. Having the buoyancy acting onto the structure where it is generated may be advantageous in one direction (supporting of horizontal risers during fabrication and installation), but a hindrance in another direction. Where two or more risers are bundled together it can be difficult to clamp buoyancy modules along each riser (due to differential thermal expansion, for example), or along one riser only (due to effective compression). To overcome this difficulty the modules are clamped to just one of the risers. However, in operation if the risers are hanging freely from the top structure of the bundle, the forces associated with weight compensation may induce a large compressive load on the riser to which the buoyancy modules are attached.
It is therefore an object of the invention to provide method and apparatus to transfer the substantial compressive forces provided by the buoyancy modules directly to the subsea structure, once installed, rather than via one or more of the risers.